The numerical aperture (NA) of an objective lens is defined as NA=n sin(.theta.), where n is the index of refraction of the image space and .theta. is the angle of the axial marginal ray (the highest ray entering the objective lens parallel to the optical axis) at the image space. Image space is defined by the location of the image surface. Thus, the numerical aperture can be increased by either increasing the angle .theta. or by immersing an image surface in a higher index medium, thereby increasing the index of refraction of the image space.
If the image space is in air (n=1), the greatest theoretically achievable NA is 1. Single element objective lenses that utilize aspheric surfaces and high index glasses can achieve numerical apertures of approximately 0.55. However, when the numerical apertures becomes larger than 0.55, optical aberrations introduced by the lens element become too great for the objective lens to be usable at such numerical apertures. Because of this problem, objective lenses with high numerical apertures (i.e., numerical apertures greater than 0.55) are constructed of two or more lens elements.
Two element high numerical aperture objective lenses have been shown to produce numerical apertures of over 0.7 and as large as 0.85. Such a high numerical aperture objective lens is illustrated in FIG. 1 and is disclosed in the article entitled "A Rewritable Optical Disk System with Over 10 GB of Capacity" by Kiayoshi Osato, Kenji Yamamoto, Isao Ichimura, Fumisada Maeda and Yataka Kasami. However, two element high numerical aperture objective lenses, such as the optical disk system of this article, are expensive to manufacture because all four lens surfaces have a different shape. These high numerical aperture objective lenses require unique manufacturing and measuring tools for every surface. In addition, during assembly of such objective lenses both kinds of lens elements need to be inventoried. Furthermore, the positions of the lens elements can not be reversed, making the assembly of such objective lenses relatively difficult and expensive.